The present invention relates generally to position sensing and, more particularly, to a system and a method for improving resolution of an encoder.
In order to control movement of a moving member, such as a rotary and/or linear actuator, it is often necessary to determine the position of the moving member relative to some frame of reference. The position can be derived as an absolute position or a relative position. The position information further can be employed to determine velocity and/or acceleration of the moving member. In some cases, velocity and/or acceleration are detected, from which desired position information is derived. The position and velocity information can be utilized to control movement of the moving member, including its travel distance, its speed, as well as other characteristics. The equipment and algorithms employed to make appropriate measurements and determine the position and/or velocity of the member usually vary depending on the environment in which the system is implemented and whether the movement of the member is rotational, such as in the case of a rotary motor, or whether movement of the member is generally linear, such as in linear motor.
For the example of a rotary motor, an encoder disk or a tachogenerator usually is secured to a rotor shaft so as to rotate with the shaft, such as during activation of the motor. An associated sensing device (e.g., optical and/or magnetic) monitors movement of the disk relative to a stator to provide an indication of the motor rotation. For a linear motor, a linear encoder scale (e.g., optical or magnetic) is arranged on a path along which a moveable stage can traverse. One or more sensors are arranged to monitor movement of the stage relative to the path based on detecting the encoder scale during relative movement between the stage and the path. The detected movement can be used to determine position and velocity of the respective motors, such as to facilitate control of the motor.
By way of further illustration, in one type of linear encoding apparatus, the sensing device is in the form of a readhead that includes a light source to illuminate a scale. Associated optical components cooperate with the scale to generate a periodic light pattern, which varies as a cyclic light intensity modulation based on relative movement of the scale and readhead. The readhead typically includes one or more photodetectors that detect modulated light intensity and emit corresponding electrical signals indicative of the light intensity modulation. The electrical signals are sent to a signal processing circuit that processes the signals to provide a pair of sinusoidally varying output signals, which may be in quadrature.
Devices of this type usually function in an incremental fashion. The output signals from the signal processing circuit serve as a basis for generating a total on a counter, indicative of the relative displacement of the scale and readhead. The total on the counter can be augmented or reduced based on the number of output signal cycles received, and the order in which these signals arrive at the counter. Additionally, to provide an increased level of resolution, interpolation can be performed relative to the output signals. Such interpolation, for example, can involve taking a ratio of the output signals from the encoder so as to compute a tangent of the angle.
In most conventional systems that employ interpolation to improve encoder resolution, such increased resolution is usually limited as a function of the speed of the moving bodies. Further, as the encoder is subject to environmental stress, such as temperature fluctuations, as well as other mechanical and electrical variables, the accuracy of most conventional systems can be compromised, especially at higher resolutions. Because most such systems also employ functional calculations to determine the interpolated position information, signal errors from the encoder signals can perpetuate through the calculations, which can result in additional inaccuracies in the computed position.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the present invention provides a system to help improve resolution of an encoder or other position sensor, which provides one or more signals indicative of relative movement between a pair of bodies (e.g., moveable parts of a linear or rotary motor). A velocity module is operative to determine a relative velocity between the bodies based on received encoder information. A low speed position module is operative to determine an indication of relative position between the moving bodies based on data stored in a position table when the system is in operating in a first mode. The operating mode is determined based on the relative velocity. The position table is populated with position data based on at least some of the received encoder information. In a particular aspect of the present invention, the encoder information can be provided as quadrature signals, with the table being populated with position data obtained between zero crossings for one or more of the quadrature signals. The system also includes a high velocity position module that is operative to provide an indication of relative position between the moving bodies as a function of the relative velocity when the system is operating in a second mode.
Another aspect of the present invention provides a method of improving encoder resolution. The method includes determining an indication of a relative velocity between the two bodies and operating between at least two different modes based on the relative velocity. A lookup table is populated based on a plurality of samples of the encoder signal obtained over a known distance. An indication of relative position between the two bodies based on position data stored in the lookup table is determined in a first of the modes. An indication of relative position between the two bodies based on the relative velocity between the two bodies is determined when in a second of the modes.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.